Light-emitting diode (LED) based illumination devices are increasingly used for a wide variety of lighting applications. LEDs offer advantages over traditional light sources, such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage and fast modulation of lumen output.
Efficient high-power LEDs are often based on blue light emitting materials. To produce an LED based illumination device having a desired color (e.g., white) output, a suitable wavelength converting material, commonly known as a phosphor, may be used which converts part of the light emitted by the LED into light of longer wavelengths so as to produce a combination of light having desired spectral characteristics. Many inorganic materials have been used as phosphor materials for converting blue light emitted by the LED into light of longer wavelengths. However, currently, organic phosphor materials are being considered for replacing inorganic phosphor in many LEDs. Organic phosphors have the advantage that their luminescence spectrum can be easily adjusted with respect to position and band width. Organic phosphor materials also often have a high degree of transparency, which is advantageous since the efficiency of the lighting system is improved compared to systems using more light-absorbing and/or reflecting phosphor materials. Furthermore, organic phosphors are much less costly than inorganic phosphors.
The main drawback hampering the application of organic phosphor materials in remote phosphor LED based lighting systems is their photo-chemical stability, which is poor. Organic phosphors have been observed to degrade quickly when illuminated with blue light in the presence of oxygen. Efforts have been made to solve this problem. For example, it has previously been found that when incorporating organic phosphors in an aromatic polyester film the lifetime of the phosphor is considerably improved. However, at operating temperatures around the glass transition temperature of an amorphous aromatic polyester the reduced dimensional stability of the polymer is a problem, which effectively limits the use of an organic phosphor in such a matrix to low intensity light source applications. To produce a phosphor film of sufficient dimensional stability the phosphor molecules may be mixed into an aromatic polyester matrix which is then biaxially oriented (stretched) and crystallized under stress, thus forming a crystalline or partly crystalline film with acceptable dimensional stability at temperatures well above the glass transition temperature of the polymer. However, it has been found that the photochemical stability of the organic phosphor is reduced in a biaxially oriented polymer as compared to the original amorphous or semi-crystalline polymer matrix. Furthermore, a biaxially oriented polymer shows a high optical transparency and addition of scattering particles is therefore necessary to achieve satisfactory outcoupling of light.
Thus, there is a need in the art for improvements with respect to the incorporation of sensitive phosphor materials in lighting devices.